The present invention relates generally to intersomatic implants which can be used in the surgical treatment of the spine.
A great many intersomatic implants are already known. These include, in particular, implants made up of several parts, particularly to give the implants certain deformability characteristics. These known implants have the disadvantage of being more expensive and difficult to manufacture, and more awkward to fit. These implants can also suffer from long-term reliability problems.
In order to overcome some of the above-mentioned disadvantages, certain implants are provided in the form of one-piece hollow bodies, or cages, having roughened areas on their upper and lower faces in order to ensure good initial immobilization relative to the overlying and underlying vertebral plates. The hollow bodies permit bone to grow through the implant for fusing the implant to vertebral bone. One such implant having a hollow body is disclosed in FR-A-2,703,580.
These known one-piece implants, despite the presence of roughened areas which become anchored in the vertebral plates, may suffer from inadequate stability in some cases. This is because the quality of the anchoring, which is effected by a simple translational movement, depends on the hardness of the bones.
Other implants have an outer body and an inner anchorage reinforcement element that is screwed into the outer body. The threads of the anchorage reinforcement element project above and below the upper and lower faces of the outer body.
One object of the present invention is to improve this type of known implant.
An implant for surgery of the spine includes an essentially hollow body insertable into an intervertebral space defined by opposing vertebrae. The body has a pair of lateral walls enclosing an internal space exposed to the overlying and underlying vertebrae. The implant also includes an anchorage reinforcement member that is screwed into the internal space of the hollow body. The anchorage reinforcement member has external threads or bone anchoring projections having a diameter greater than the overall height of the body. The external threads may be self-tapping screw threads or have a generally square radial cross-section.
To counter the reverse movement of the body out of the intervertebral space, and hence to further improve the securing of the implant in position, the hollow body has upper and lower surfaces provided with sharp-edged teeth which can be anchored in the vertebrae. The teeth preferably have a triangular cross-section.
According to another preferred embodiment of the present invention, in order to improve the compactness of the implant and to make it easier to put into place, the lateral walls of the hollow body are partially cylindrical and coaxial with an axis of the anchorage reinforcement member.
According to yet another preferred embodiment of the invention, the lateral walls have through-openings permitting bone to grow through them. These through-openings preferably include elongate slots extending substantially parallel to the direction of insertion of the anchorage reinforcement member into the hollow body.
In order to give the body a greater width, it is possible to use, for the lateral walls of the body, thick walls in which second through-openings are formed, extending between the upper and lower faces of the body. Bone growth can also be induced between the two vertebral plates via these second through-openings.
The first through-openings preferably bring the internal space into communication with the second through-openings.
It is also possible to provide through-openings which bring the second through-openings into communication with the outer sides of the body.
According to another preferred embodiment of the invention, the hollow body has a distal end wall connecting the lateral walls. The distal end wall is rounded to facilitate insertion of the hollow body into the intervertebral space.
The invention also preferably includes an implant as described above, in which the hollow body has a distal end wall connecting the lateral walls. The distal end wall has a tapped hole for temporarily fixing the hollow body to an instrument for facilitating insertion of the body.
This screw thread of the anchorage reinforcement member preferably has a radial cross-section which changes progressively from an essentially triangular radial cross-section to the generally square radial cross-section starting from the distal end of the thread, whereby the diameter of the screw thread increases progressively, starting from its distal end, up to a part of essentially constant diameter.
According to another preferred embodiment of the invention, an implant is also proposed in which the projections for bone anchorage comprise a screw thread in the form of a helical band encircling an internal space of the anchorage reinforcement member.
The helical band is advantageously connected to a fork extending inside the band in an axial direction of the member, and this fork preferably comprises two branches extending from a proximal end wall of the anchorage reinforcement member.
The fork may also include branches having a frustoconical external surface, the diameter of which decreases from the proximal end towards the distal end of the member. This makes it possible to compress a substance promoting bone growth, placed beforehand in the anchorage reinforcement member, when the latter is being screwed in.
Alternatively, the fork includes at least two branches, each having a cutting edge for cutting or biting into bone, in order thereby to accumulate bone chips inside the member 20 and facilitate bone fusion. It is preferable for the fork and the helical band to be made in one piece.
The invention also proposes an implant in which the projections for bone anchorage include a screw thread, whereby the anchorage reinforcement member has structure for immobilizing the latter against reverse rotation. The immobilizing structure preferably includes a deformed part of the thread in the region of its proximal end, which improves the stability of the implant until fusion has taken place.
According to another preferred embodiment of the present invention, the projections for bone anchorage include a screw thread having an external diameter which decreases from the proximal end toward the distal end in order to make it easier for the screw thread to penetrate the vertebral plates.
In another preferred embodiment, the anchorage reinforcement member has a proximal end wall which is able to essentially close a frontal opening of the hollow body in such a way that a substance for promoting bone growth, placed inside the member, is compressed during the insertion of the member into the hollow body.
In this embodiment, the anchorage reinforcement member has at least one part whose external surface belongs to a truncated cone. Alternatively, the anchorage reinforcement member is substantially shorter than the body and has a generally conical point directed towards the distal end wall of the body.
It is advantageous in this case that the proximal end wall of the member has a tapped opening for temporarily fixing the member to an instrument for inserting the member.
According to yet another preferred embodiment of the present invention, the anchorage reinforcement member has indexing means for fixing the member to an instrument for inserting the member in a given angular relationship.
According to another preferred embodiment, the anchorage reinforcement member has a plug attached at its proximal end. The plug can, for example, be screwed into a tapped frontal opening of the anchorage reinforcement member, or else can be engaged by being clipped elastically into a frontal opening of the anchorage reinforcement member.
The plug preferably has an arrangement which can cooperate with an instrument allowing the member to be driven in rotation, and/or arrangements for angular indexing of the anchorage reinforcement member with an instrument for positioning the member.
It is also proposed according to the invention that the projections for bone anchorage include a screw thread, and that at least one of the lateral branches of the body has a reentrant part forming a threading which is able to cooperate with the thread. This reentrant part can be provided only on one of the branches and can constitute the only part of the body cooperating, by screwing, with the screw thread. In addition, this reentrant part can have an essentially rectilinear free end edge.
According to another preferred embodiment, the invention proposes an implant in which the member is oriented obliquely, for example at about 45xc2x0, in relation to a plane of the body corresponding to the sagittal plane.
According to another preferred embodiment, the anchoring member has through-openings provided in the member between the interior and the exterior thereof. The openings are elongate in an essentially circumferential direction of the member.
According to another preferred embodiment of the present invention, the body has a distal end wall, and the anchorage reinforcement member has a distal end part which can be screwed into an opening of the distal end wall.
The body can also have a proximal end wall including an opening which is wider than the external dimension of the anchorage reinforcement member and in which the member can be engaged freely.
The invention furthermore proposes an implant in which the body has a proximal wall, a distal wall and two lateral walls, the walls defining therebetween an internal space which is larger than the anchorage reinforcement member. This assembly increases the space designated for bone growth between the overlying and underlying vertebral plates.
The anchorage reinforcement member may have a threaded part for screwing it into the proximal wall of the body. In other preferred embodiments, the anchorage reinforcement member and the distal wall of the body may have threaded structure cooperating with each other for fixing the member to the body.
In such a configuration, the projections for bone anchorage can comprise a screw thread having the same pitch as the threaded part or the threaded means for fixing to the body.
The shape of the body, in this case, is preferably such that the lateral walls and the proximal wall of the body extend essentially on the same arc of a circle, and that the distal wall is essentially rectilinear.
It is also advantageous that the upper and lower faces of the body have projections for bone anchorage which extend along its walls.
According to another preferred embodiment of the invention, an implant is provided with mounting structure for mounting the anchorage reinforcement member so that it can rotate in the internal space of the body, while at the same time preventing relative translational movement between the hollow body and the anchorage reinforcement member.
The mounting structure preferably includes a cylindrical opening formed in a distal end wall of the body, and a shaft provided on the member and able to be engaged, by elastic deformation, in the opening.
In this particular embodiment, the anchorage reinforcement member preferably has the shape of a screw having two diametrically opposite flats, with the projections for bone anchorage being defined between the flats, and cutting edges being provided at the transitions between the thread of the screw and the flats in order to promote bone fusion once the implant has been put into place.
To facilitate the insertion of the implant, the distance between the opposite flats is not greater than the distance between the upper and lower faces of the body.
According to another preferred embodiment, of the invention, an implant for surgery of the spine includes an essentially hollow body which can be inserted into an intervertebral space, the body having a group of generally parallel walls defining at least two internal spaces situated side by side and exposed to the overlying and underlying vertebrae which define the intervertebral space. The implant additionally has at least two anchorage reinforcement members having, on their external surface, projections for bone anchorage having a diameter greater than the overall height of the body, the said anchorage reinforcement members being adapted to be threaded into respective internal spaces of the body.
In this particular embodiment, the anchorage reinforcement members are preferably identical. The invention furthermore proposes that the hollow body can have different geometries. In one preferred embodiment, the upper and lower surfaces of the hollow body are inclined in relation to one another, with a distance between them which decreases from the proximal end towards the distal end of the body. In another preferred embodiment, the upper and lower surfaces of the hollow body are inclined in relation to one another, with a distance between them which decreases from a first lateral side of the body towards the opposite lateral side.
The invention furthermore proposes a set of implants for forming a spinal implant intended to be inserted into an intervertebral space of the human vertebral column by being adapted to the geometry of the intervertebral space. This set of implants includes a plurality of hollow bodies, each having a pair of lateral walls defining an internal space and each able to be inserted into an intervertebral space in such a way that the internal space is exposed to the overlying and underlying vertebrae which define the intervertebral space. Each of the hollow bodies preferably has a specific size and shape, at least one anchorage reinforcement member having, on its external surface, projections for bone anchorage having a diameter greater than the overall height of the bodies. The anchorage reinforcement member is able to be driven in rotation in the internal space of any one of the bodies, in such a way that a specific hollow body appropriate to the particular configuration of a given intervertebral space can be chosen from among the plurality of hollow bodies.
The specific sizes and shapes of the bodies may vary. Certain preferred bodies have different angles of inclination between their upper and lower surfaces. Other preferred bodies have different widths. The widest hollow bodies preferably have lateral walls in which through-openings are formed which extend between the upper and lower faces of the bodies. The hollow bodies may also have different heights and different lengths.
Regardless of their size or shape, the hollow bodies are preferably adapted to receive the same type of anchorage reinforcement member.
The invention also proposes a method for positioning, in an intervertebral space of a human vertebral column, an implant including an essentially hollow body having a pair of lateral walls enclosing an internal space, and an anchorage reinforcement member having, on its external surface, projections for bone anchorage having a diameter greater than the overall height of the body, and adapted to be driven in rotation in the internal space of the body. The method preferably includes selecting a hollow body from a set of hollow bodies having different shapes and dimensions. The selected hollow body is preferably adapted to fit with the configuration of the intervertebral space. The method also includes filling the selected hollow body with a substance which promotes bone growth, pushing the hollow body into the intervertebral space in such a way that the internal space thereof is exposed to the overlying and underlying vertebrae which define the intervertebral space, and inserting the anchorage reinforcement member into the hollow body in such a way that the projections of the bone anchorage member anchor in the overlying and underlying vertebrae.
In embodiments where the intention is to position an implant whose body has lateral walls provided with through-openings extending between the upper and lower faces of the body, the method can additionally include, before the step of pushing the hollow body into the intervertebral space, a step in which the through-openings are filled with a substance which promotes bone growth.